In general, polyester, particularly, polyethylene terephthalate (hereinafter, referred to as “PET”) is used in film, fiber, a container, a bottle, or mechanical and electronic components due to advantages thereof, such as, excellent heat resistance, mechanical strength, transparency, chemical resistance, and the like, and the use and use amount thereof are expanding due to lower cost thereof as compared with other high-performance resins. Particularly, the polyester film that is technologically manufactured presently has been widely used as a base film for a magnetic recording medium, materials for various kinds of packages, and other industrial uses. In recent, the markets thereof are expanding based on an optical film together with the development of various kinds of display electronic products.
The optical films used in the recent display electronic products, for example, a prism sheet, a light diffusing sheet, a base film for a touch panel, and the like, require superior luminance and sharpness, with the technical development, and thus technologies for minimizing an internal defect and surface defect hindering excellent transparency and smoothness are needed.
Here, when the term defects is defined in only the present invention, the internal defect is referred to as a factor that is present inside the PET and it (internal defect) has a different refractive index, thereby inducing reflection and scattering of light, resulting in deteriorating transparency of the PET. The internal defect may be caused from inorganic metals, external particles, particle agglomeration, carbide, and the like. In addition, the surface defect is referred to as scratch, surface roughness, or the like, which is present in a surface of a PET film to thereby induce reflection and scattering of light and cause problems in a subsequent process.
In most polyester resins represented by PET, an antimony compound is used as a main catalyst in a manufacturing process (particularly, a polycondensation process). However, the antimony compound has a disadvantage in that the compound is present as defects inside the PET after manufacturing resins even though the antimony compound is well dissolved in ethylene glycol at a high temperature. In addition, it has been recently found that the antimony compound is harmful to humans, and as a result, it is determined that the use of the antimony compound is much restricted.
In addition, in a film manufacturing process using a general polyester resin, particularly a casting process, high voltage of electricity is applied to the pinning wire to generate strong static electricity (charges), to closely adhere a sheet type of melted polymer resin discharged from a die on a casting drum, thereby preventing vapor from being sucked between the polymer resin and the casting drum and improving a cooling effect. In order to achieve this, as described in Korean Patent Laid-Open Publication No. 10-2009-0069632, the polyester resin for a film includes a pinning additive for generating charges during a manufacturing process, particularly an alkali metal compound or an alkali earth metal compound, and further include a phosphorous compound, as some of the constituent components. Also, in the related art, internal particles by this metal compound and phosphorous compound are precipitated by a polycondensation reaction system, and thus is utilized to improve high-speed drivability or scratch resistance of a proclaim film or the like.
However, the metal compound and the phosphorous compound react with bis-2-hydroxyethyl terephthalate (BHT) generated during a PET polymerization process (particularly, esterification process), to thereby form a BHT-P Complex and a BHT-Metal-P Complex, and form a complex with antimony used as a catalyst. These complexes are left as defects in the PET after manufacturing the resin, and thus become a fatal disadvantage in being used for a high-grade optical film.